View Full Version : From the "This can't work! It's too simple!" department, running stepper motors on AC

06-24-2008, 01:07 PM
I stumbled on this "circuit" online last night and tried it. It actually works. If you have a box full of old steppers and wish you could use them as ordinary motors for whatever, you can. It runs the stepper as a synchronous motor by using a capacitor that produces a roughly 90 degree phase angle change in the sine wave to one of the windings. Calculating the synchronous rpm is easy. For a 7.5 degree per step motor it's 300 rpm.

360 degrees / 7.5 degrees per step=48 steps per rev
48/4 steps per cycle =12 cycles per rev
3600 cycles per minute/12 =300 rpm

I have tried it with three very different motors and they all worked. The capacitor must be an electrolytic non-polarized type. If you don't have one you can connect two polarized capacitors of twice the value back to back so that both positives are connected together and the negatives become the leads of the non-polar capacitor.

I used a 12vac centre tapped xformer for the test and found the smaller two motors ran best on six volts while the large motor ran only on 12vac. The torque isn't great but it is certainly enough to be useful. By experimenting with the capacitor value you can change the phase angle to produce a preferred starting direction. If the phase angle is very close to 90 degrees it will randomly start either direction.


Forrest Addy
06-24-2008, 01:59 PM
So-o-o-o the next step is to run one from a stereo amplifier using a variable oscillator input. . Buy using the bass control you can tailor the output to increase in ampliture with frequency so the step motor has constant torque.

Bada-bing! You cont a constant current variable frequency drive for a step motor from a yard sale stereo. Oh yeah, phase shift. Use an R/C phase shift at the input and the balance control to tweak it into the same amplitude as the direct signal.

Quick and dirty.

Quetico Bob
06-24-2008, 03:31 PM
What about vibration? From what I have read this can be severe at certain speeds contributing to even more torque loss. Have you noticed any of this in your experiments.
Cheers, Bob

06-24-2008, 05:02 PM
Superior Electric used to label their Steppers something like "Stepping/Synchronous Motor" And they had cap ratings on them as well.

06-24-2008, 05:07 PM
I have only played with it for a few minutes. Because it's a sine wave they are pretty smooth. That also means the average power is less than a square wave and much less than a chopped square wave. However, you can get back a lot of the lost power by using a much higher voltage (24vac?) and a series resistor to limit the current. The higher voltage creates a shorter time constant vs the motor inductance so the magnetic field builds faster. Of course the capacitor needs to be rated for the higher voltage as well.

J Tiers
06-24-2008, 09:42 PM
The capacitor must be an electrolytic non-polarized type.

Or a non-electrolytic motor run type.

In fact, you just invented the capacitor run motor, in a way......

06-24-2008, 10:44 PM
You can also add a switch to reverse direction.

06-25-2008, 02:56 AM
So, what do you do with a stepper motor that has six leads instead of four?


06-25-2008, 04:34 AM
Ignore two of them. They are center taps and as you can see on the one that is hooked up they are floating to the right.

I haven't given this much thought yet as I am busy with other things but the principle isn't limited to just driving a stepper from AC. All that is needed to drive one is a signal in quadrature and that can very easily be generated by a simple analog circuit composed of a pair of op amps. If they are powered by a split supply there are many op amp types that can directly drive small steppers for use as variable frequency synchronous motors.

Here is an example circuit. This is ground referenced with a single ended supply but that is easy to change.


reference data sheet here: http://www.calvin.edu/~pribeiro/courses/engr332/Handouts/oscillators.pdf

06-25-2008, 04:29 PM
Yup. "Slo-Syn Synchonous/Stepper" I think they were a stepper motor that had mechanical characteristics optimized for running on AC. I have one lying around here someplace.

I had one of their skinny databooks from the mid-70s and it's probably the easiest to read tutorial on steppers I have ever seen.

Superior Electric used to label their Steppers something like "Stepping/Synchronous Motor" And they had cap ratings on them as well.